Description:
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Current implementation of OpenSSL functions has openssl_cipher_iv_length, obtaining the initialize vector length for ciphers, but no way to get key length. 
Moreover, in the function openssl_encrypt, the argument $password is used simply as key and cut off if longer than algorithm specified max key length (see below example). 
Then it is insecure to pass the raw password to openssl_enctypt, so we want to apply PBKD; Password Based Key Derivation, in php implemented as Hash functions (hash_pbkdf2), but this algorithm requires the key length.
This is why we cannot migrate from mcrypt to openssl (in mcrypt functions, mcrypt_get_key_size is defined).

To summarize the above, we need the way to get max key length for cipher algorithms, like openssl_cipher_key_length. 

Test script:
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echo(openssl_encrypt('aaa', 'aes-256-cbc', str_pad('', 256, '0')).PHP_EOL);
echo(openssl_encrypt('aaa', 'aes-256-cbc', str_pad('', 300, '0')).PHP_EOL);

// These two code pass different passwords to openssl_encrypt but get same result.